Smallest Amount of Force Ever Measured

6/28/2014

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A group of scientists led by
Dan Stamper-Kurn of UC Berkeley have proclaimed that they have measured the least
amount of force so far. By means of a blend of lasers and optical trapping,
they were able to identify 42 yoctonewtons. One yoctonewton is a septillionth
i.e. 1/1024 of a Newton. However researchers once supposed the standard quantum
limit (SQL) would be touched in 30 years ago; earlier measurements within latest
years have only come within a figure of 6-8 above SQL. These outcomes come just
a factor of 4 greater than the SQL. The group’s paper was published in Science.
Stamper-Kurn said in a press release “We applied an external force to the
center-of-mass motion of an ultracold atom cloud in a high-finesse optical
cavity and measured the resulting motion optically. When the driving force was
resonant with the cloud’s oscillation frequency, we achieved a sensitivity that
is consistent with theoretical predictions and only a factor of four above the
Standard Quantum Limit, the most sensitive measurement that can be made.”

The Standard Quantum Limit is enforced by the Heisenberg uncertainty principle, in which the measurement itself disturbs the motion of the oscillator, an occurrence recognized as “quantum back-action. Image by Kevin Gutowski

The SQL is carried about by
the Heisenberg uncertainty principle, which fundamentally states that two free
measurements (which are force and motion, in this situation) can only get so accurate
without negatively disturbing one another. In calculating the force optically,
the apparatus used will get to a point where they are disturbed more by the action
of measuring than by the object it is supposed to be record, because of
“quantum back-action.” The SQL is utmost exact measurement of quantum force
that can be taken before calculations are skewed. The group used a mechanical
oscillator generated by two standing-wave light fields at marginally different
wavelengths in order to optically trick a cloud of just 1,200 rubidium atoms.
After the cloud was carried down close to absolute zero, one of the light waves
generates a force on the cloud by faintly varying the wave’s amplitude. A laser
examination is then able to measure the tiny sum of force. Lead author Sydney
Schreppler explains “When we apply an external force to our oscillator it is
like hitting a pendulum with a bat then measuring the reaction. A key to our
sensitivity and approaching the SQL is our ability to decouple the rubidium
atoms from their environment and maintain their cold temperature. The laser
light we use to trap our atoms isolates them from external environmental noise
but does not heat them, so they can remain cold and still enough to allow us to
approach the limits of sensitivity when we apply a force.”

By means of colder atoms and
more progressive optical equipment, the scientists are assured that they can sense
forces even nearer to SQL. There are also various methods that can be tried in
order to dodge the quantum back-action that generates noise when measuring
force. Improvements on this front could also direct to enhance atomic force
microscopy.

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